Schedule of Common Stock Repurchases (Detail) - 2015 Repurchase Program - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018
Schedule Of Common Share Purchase [Line Items]
Shares repurchased	368,551	403,623
Cost of shares repurchased (in millions)	$ 214.8	$ 236.1
Average price per share repurchased	$ 582.92	$ 584.96

Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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